What is the number of members of the Booster Club who are under 21

What is the number of members of the Booster Club who are under 21 years old?

(1) Exactly 2/3 of the members of the Booster Club are at least 21 years old.
(2) The 15 males in the Booster Club account for 50 percent of the club's membership.

To find : Number of member under 21 yr ?

St 1 --> we only get 1/3 of total member are under 21 yrs . Not sufficient as we don't the total member of the club .

St 2 --> 50/100 ( total member ) = 15
hence total member = 30
but not info given on under 21 yrs old . hence not sufficient

Together ...
We get all the info Hence C ans .

Forget conventional ways of solving math questions. In DS, Variable approach is the easiest and quickest way to find the answer without actually solving the problem. Remember equal number of variables and independent equations ensures a solution.

What is the number of members of the Booster Club who are under 21 years old?

(1) Exactly 2/3 of the members of the Booster Club are at least 21 years old.
(2) The 15 males in the Booster Club account for 50 percent of the club's membership.

This is a "2by2" question, one of the most common type of questions in GMAT math

There are 4 variables (a,b,c,d) but only 2 equations are given by the conditions, so there is high chance (E) will be the answer.
Looking at the conditions together,
a+b=15, c+d=15, and b+d=30(2/3)=20, so a+b+c+d=30 and from a+c+b+d=30, a+c+20=30, a+c=10, so we can derive an unique answer, and the conditions are sufficient and the answer becomes (C).

For cases where we need 3 more equations, such as original conditions with “3 variables”, or “4 variables and 1 equation”, or “5 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 80% chance that E is the answer (especially about 90% of 2 by 2 questions where there are more than 3 variables), while C has 15% chance. These two are the majority. In case of common mistake type 3,4, the answer may be from A, B or D but there is only 5% chance. Since E is most likely to be the answer using 1) and 2) separately according to DS definition (It saves us time). Obviously there may be cases where the answer is A, B, C or D.

The question asks to find the number of people whose age is less than 21. Let the number of members whose age is less than 21 be x.

So x=?

Statement 1: Exactly 2/3 of the members of the Booster Club are at least 21 years old.

So? this gives y (number of members with age more than or equal to 21) ---> y>= (2/3) * total members. But no value of total members. Hence, Insufficient

(2) The 15 males in the Booster Club account for 50 percent of the club's membership.

So total number of members = 30. But what percentage of these has ageless than 21 years old. NO info. Hence, Insufficient

Combining the two statements, we get total number of members and fraction of people with age more than or equal to 21. Remaining has age less than 21.

Hence, C